As I remember it. Hutch McKinney was fired from Southfork Ranch by Jock Ewing, and Rebecca Barnes was then going to run off with Hutch later that day, taking young Cliff with them. Digger Barnes came home drunk, wondered what was going on, Rebecca said that she was leaving Digger for Hutch and that the unborn baby (Pam) was Hutch's, angering Digger. Hutch then pulled a gun (stolen from Jock) on Digger. Rebecca physically disarmed Hutch, trying to calm down the situation, but Digger picked up the gun and shot Hutch dead in a temper. Digger then buried the body on Southfork land, and became Pam's legal father. Rebecca left Digger when Pam was around 1 year old, presumably by faking her death so that Digger wouldn't track her down. Cliff and Pam were brought up from then on by their Aunt Maggie (Digger's sister) and Digger when he wasn't wandering and drunk.
Who is Pam's father? Valene gave a good speech to her son Bobby in Knots Landing about this. Val said that there were two kinds of father, one who helps to make the baby and one who helps to bring up the baby, that they are usually the same man but not always. Digger is Pam's legal father, and Pam grew up until Digger's death believing Digger was her father. In the flashback, Digger seemed to believe Rebecca's unborn baby to be his until he was told different. Hutch was dead before Pam was born, and there was never a DNA test on who Pam's father was anyway. I also remember they said that Hutch was fired from Southfork soon after Eisenhower was first elected US President (late 1952), which is before DNA's official discovery. Hutch was the bloke that Rebecca was about to run off with, so it fits for Rebecca to say that Hutch is the father of her unborn baby. For all we know, Rebecca didn't know who Pam's father was.
So, for all intents and purposes, Digger is legally Pam's father. If you do a family tree, Digger would be down as Pam's father. I can also understand why Pam would feel a certain emptiness about her background after Digger's deathbed revelation, though.
Changing Pam's biological father was ridiculous. I still say it was the worst mistake of pre-1984 Dallas by some distance. I'm still wondering why they did it, particularly with them having a perfect storyline excuse (neurofibromatosis) for Pam's future storylines of not having a biological baby of her own.